The present invention relates to a protective system for a high voltage transmission line, and more particularly, to an improved reactance type protective relay.
Protective systems for high voltage transmission lines commonly employ a protective relay as part of that protective system for detection of fault conditions within a protected portion of the transmission line. A reactance type protective relay may be coupled to a particular phase A, B, or C of the transmission line or a phase pair AB, BC, or CA of the transmission line. However, because substantial fault resistance is more common for faults involving ground, the reactance relay is typically employed to measure the impedance between phase and ground.
The operating characteristic of the reactance type relay is typically specified in terms of a resistance (R) and reactance (X) diagram and the characteristic of the reactance type relay is typically displaced from and in parallel with the R axis of the diagram.
It has been the practice in some applications to use an angle impedance characteristic, rather than a typical reactance characteristic, which is tilted toward the R axis, thus permitting operation on a decreasing reactance magnitude as the resistive component of the fault impedance increases.
It has also been the practice in some applications to use a sequence component of the phase current that does not include load current, such as the negative or zero sequence component, for polarizing the reactance relay. The primary effect of the sequence components is that the load flow can be eliminated from causing the relay current to have a different phase angle compared to the fault current. However, the different impedance angles from the fault to the source on either side of the fault may still cause an apparent reactive component in the fault resistance. The apparent reactive component that may occur can be difficult to evaluate since the source impedance angle may change with system switching. Further, if there is disymmetry in the power system, there will probably be negative and zero sequence currents generated as a consequence of load current flowing through the disymmetry that will cause even greater difficulty in assessing the apparent reactive component in the fault resistance.
It should be noted that the apparent reactive component in the fault resistance can be positive or negative, causing the reactance relay to underreach or overreach. The terms "underreach" and "overreach" are related to the reach of the relay. The reach of the relay is the reactive component of the impedance between the relay and the fault for which the relay is set to operate.
Overreaching results when the reactance from the relay to the fault is larger than the reach setting of the relay, but the addition of the apparent negative reactance in the fault impedance causes the net reactance seen by the relay to be less than the reach setting. Conversely, the reactance between the relay and the fault may be less than the reach setting of the relay, but the addition of an apparent positive reactance in the fault impedance could cause the total reactance seen by the relay to be greater than the reach setting of the relay, thus causing "underreaching".
If the security, (that is, security against misoperation of the protective relay), is very important, then the relay characteristic may have to be tilted substantially towards the R axis (with increasing fault resistance magnitude) to ensure that overreach does not occur for the worst combination of the factors mentioned above. Thus, substantial underreach may result if the combination of the factors mentioned above contribute to underreach, as well as the underreach obtained by tilting the characteristic of the angle impedance relay. If dependability (that is the ability to trip on internal faults) is more important, then the reactance characteristic is used, but with the risk of false tripping due to overreach.
Accordingly, it is an object of the present invention to provide an improved reactance type relay having means to discriminate against overreaching and underreaching along the transmission line so as to provide a highly reliable (in both security and dependability) protective relay.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following description of the invention.